Wireless communications systems are used in a variety of telecommunications systems, television, radio and other media systems, data communication networks, and other systems to convey information between remote points using wireless transmitters and wireless receivers. A transmitter is an electronic device which, usually with the aid of an antenna, propagates an electromagnetic signal such as radio, television, or other telecommunications. Transmitters often include signal amplifiers which receive a radio-frequency or other signal, amplify the signal by a predetermined gain, and communicate the amplified signal. On the other hand, a receiver is an electronic device which, also usually with the aid of an antenna, receives and processes a wireless electromagnetic signal. In certain instances, a transmitter and receiver may be combined into a single device called a transceiver.
In many modern wireless communication systems, it may desirable to transmit wireless signals at multiple frequencies or “bands.” Traditionally, transmitters include multiple transmit chains (essentially, multiple transmitters) in order to support transmission at multiple frequencies. Traditional transmitters often used this approach as separate transformers were required for each frequency. Transformers used in transmitters are often integrated on a semiconductor chip (e.g., in a CMOS process), and thus may be referred to as integrated transformers.
A transformer is a device that transfers electrical energy from one circuit to another through inductively coupled conductors—the transformer's coils—via a phenomenon known as mutual induction. With mutual induction, a varying current in a primary winding of a transformer creates a varying magnetic flux in a core of the transformer about which the windings are wound, and thus a varying magnetic field through the secondary winding. This varying magnetic field induces a varying electromotive force (EMF) or voltage in the secondary winding. If a load is connected to the secondary, an electric current will flow in the secondary winding and electrical energy will be transferred from the primary circuit through the transformer to the load. In an ideal transformer, the induced voltage in the secondary winding is in proportion to the primary voltage, and is given by the ratio of the number of turns in the secondary to the number of turns in the primary.